Basic Tutorial - XML to JSON in Post Flow

[#1]

Hi - I am just starting out with Apigee and am trying to run the basic proxy tutorial - when applying XML to JSON as a post flow attribute I end up getting the following -

{"fault":{"faultstring":"XMLToJSON[XML-to-JSON-2]: Source response is not available","detail":{"errorcode":"steps.xml2json.SourceUnavailable"}}}

http://docs.apigee.com/api-services/tutorials/add-policies-your-api